1. Field of the Invention
Embodiments of the present invention relate, in general, to psychoacoustic bass enhancement and more particularly to psychoacoustic bass enhancement using asymmetric polynomial distortion.
2. Relevant Background
Due to limitations imposed by size, power and cost, the speakers used in televisions, personal computers, notebooks, portable devices etc., often have poor response at the lower part of their frequency range and as a result their reproduction of bass sounds is poor. For example, television manufacturers often cut costs and save space by using small, inexpensive speakers with poor low-frequency response. FIG. 1 shows a typical amplitude response of a small speaker. As shown in FIG. 1, small speakers often do a poor job of reproducing signals with frequency lower than 100 or 150 Hz, called “deep bass” signals. The frequency response 110 of the speaker output has extreme attenuation at the deep bass frequencies 120. The inability of the speakers to reproduce bass content in an audio signal drastically diminishes the auditory experience of the listener, and brute-force attempts to amplify the bass level can result in loudspeaker overload and unpleasant distortion.
Because such speakers are very inefficient at low frequencies, attempts to increase the bass volume by amplifying the low frequencies can result in high cone excursion, which may overload the speakers and cause unpleasant (raspy or rattling) distortion. Therefore, other ways for improving the sound quality have been considered.
One means to enhance the bass content is by making use of psychoacoustic properties of the human hearing system. Psychoacoustic bass enhancement produces deep bass signals by non-linearly processing the audio signal. The bass frequency reproduction capability of limited frequency range speakers is improved by preprocessing the audio signal to generate harmonics of the fundamental bass tones.
Psychoacoustic studies have shown that, when subjects listen to only the harmonics of a bass signal, they perceive the fundamental frequency, even though the fundamental frequency is missing. This “missing fundamental effect” is exploited to create an auditory experience of hearing deep bass content using speakers that lack the ability to produce deep bass.
One psychoacoustic technique known in the art is to use a set of filters to derive the deep bass signal and to bandlimit the non-linearly processed deep bass signal, as shown in FIG. 2. In FIG. 2, the left input 210 and right input 220 are summed 225 and sent to first bandpass filter 235, thus extracting a bass signal, x(t), while rejecting sub-audio signals. The left input 210 and the right input 220 independently pass through a high pass filter 230. A non-linear distortion 240 is applied to the bass signal, generating a non-linearly processed bass signal, ŷ(t), containing harmonics of the fundamental frequency. The non-linearly processed bass signal is sent to a second bandpass filter 260, scaled by the Gain 265 and added 225 to the highpass filtered 230 left and right inputs 210, 220 to produce left and right outputs 250, 260. A variety of similar structures can be used instead.
Since low frequencies cannot be reproduced effectively by the speakers, it is desirable to apply non-linear processing to create harmonics that will give the illusion of the missing fundamental. A variety of non-linear processing methods have been used in the past with limited success, such as “harmonics generation by multiplication,” rectification, and integration (involving integrating a rectified input signal and resetting the output to zero after alternate zero crossing).
The rectification method only produces even harmonics, which causes an undesirable doubling of the perceived pitch; it also produces a great deal of undesired intermodulation distortion. The integration method has poor temporal characteristics, with a slow attack and decay. And the “harmonics generation by multiplication” method can only produce a fixed, finite number of harmonics, and it produces a variable amount of intermodulation distortion, depending on the harmonic number.
A challenge therefore remains to provide a non-linear process supportive of psychoacoustic bass enhancement to create even and odd harmonics that provide a realistic illusion of missing fundamental frequencies without the deficiencies and inadequacies of techniques currently known in the prior art. These and other challenges of the prior art are addressed by one or more embodiments of the present invention.